The present invention relates generally to a magnetic recording medium and to a method of producing the same, and particularly to a magnetic recording medium with high areal recording density that has excellent thermal fluctuation resistance characteristics and an enhanced signal-to-noise ratio (S/N ratio), as well as to a method of producing the same.
Because of their random access capabilities, magnetic disks have been widely used as external information storage media for computers and the like. In particular, Hard Disk Drives (HDD) using a hard material such as aluminum or glass as a substrate have been widely used because of their excellent response properties and large storage capacity. Particularly, in magnetic disk devices for mobile personal computers with a diameter of 2.5 inches, glass substrates that are high in hardness and less liable to plastic deformation are widely used with the main purpose of providing improvements in shock resistance. Additionally, in magnetic disk devices for desk-top personal computers, very smooth glass substrates (resulting from their high hardness) have gradually come to be used in place of aluminum substrates.
In magnetic disk devices, various approaches have been tried to enhance the areal recording density. When enhancing the areal recording density of magnetic recording media, it is also necessary to reduce the medium noise and to enhance the signal-to-noise ratio (S/N ratio). Some of the most effective methods of reducing the medium noise include refining and uniformizing magnetic particles, which are the minimum recording elements in the magnetic recording media, and magneticably isolating the magnetic particles.
However, with advances in refining and isolating the magnetic particles, there arises the problem of an increase in a phenomenon in which magnetic information recorded on the magnetic recording medium is attenuated due to external thermal energy. Such a phenomenon is called the thermal fluctuation effect or thermal decay.
The magnitude of the thermal fluctuation effect is generally considered to be determined by the magnitude of KUV/kBT, and the thermal fluctuation becomes more conspicuous as the value of KUV/kBT decreases, where KU is a magnetic anisotropy constant, V is the average volume of the magnetic particles, kB is Boltzmann""s constant, and T is temperature. The value of KUV/kBT is also called the thermal stabilization constant.
Japanese Patent Laid-open No. 2001-56924 and Japanese Patent Laid-open No. 2001-148110 disclose examples of mediums that have thermal fluctuation resistance with an enhanced S/N ratio. These magnetic recording mediums each include an exchange layer structure composed of a nonmagnetic coupling layer and a ferromagnetic layer, in addition to a magnetic recording layer.
In a magnetic recording media with a glass substrate, in order to provide a medium having a high coercive force (Hc), the substrate temperature must be maintained at a high value when forming the magnetic film. Japanese Patent Laid-open No. Hei 5-197941 (1993) and Japanese Patent Laid-open No. Hei 11-339240 (1999) each disclose a technique of sequentially forming a Cr adhesive layer, an NiP layer, a Cr underlayer and a magnetic recording layer on a glass substrate.
In addition, in response to the demand for increasing the recording density and decreasing the flying height of the magnetic head, there is also known a technique of subjecting an NiP layer to a mechanical texturing treatment after forming the NiP layer on a glass substrate by sputtering. The main purposes of such a procedure are to refine the magnetic particles, enhance the magnetic anisotropy, and prevent stiction of the magnetic head.
Thus, when using a glass substrate as a substrate, it is the general practice to form a Cr adhesive layer and an NiP layer on the glass substrate, and to then subject the surface of the NiP layer to a texturing treatment. However, in order to conduct the texturing treatment after formation of the NiP layer, it is normally necessary to first remove the glass substrate from the vacuum film-forming chamber of the sputtering device and bring it into the atmosphere prior to conducting the texturing treatment. On the other hand, when growing the NiP layer on the glass substrate by a plating method, the surface of the NiP layer necessarily comes into contact with the atmosphere and is oxidized in the process of moving the substrate into a vacuum chamber for forming the magnetic recording layer.
Thus, in each of the above cases, according to the conventional methods of producing a magnetic recording medium, the production process is divided into two processes. Namely, a first process of the steps up to and including forming the NiP layer on the glass substrate, and a second process of forming the magnetic recording layer. Generally, after the first process is completed, the medium is removed from the first chamber (at which point the medium comes into contact with the atmosphere), and it is moved into a second chamber. Thus, the production equipment and production costs are higher than they would be if all of the steps of forming the magnetic recording layer on the substrate could be accomplished within a single chamber in which the vacuum is maintained.
On the other hand, in a magnetic recording medium in which the layers from the NiP layer to the magnetic layer are successively formed on the glass substrate in a vacuum film-forming chamber, as performed in a trial investigation conducted by the applicants, good magnetic characteristics are secured to a certain extent, but the electromagnetic transformation characteristic obtained is much inferior to that of a magnetic recording medium produced through a production process divided into the above-mentioned two processes.
It is known that the above result is due to the fact that, in the case of the conventional magnetic recording medium, the glass substrate is taken out of the vacuum film-forming chamber to subject the surface of the NiP layer to the texturing treatment and, as a result, the surface of the NiP layer is oxidized, whereby the electromagnetic transformation characteristic is improved.
Accordingly, an object of the present invention is to provide a magnetic recording medium that has excellent thermal fluctuation resistance, has an enhanced S/N ratio of reproduction signals and is suitable for high areal recording density recording, as well as a method of producing the same.
In accordance with one aspect of the present invention, there is provided a magnetic recording medium that includes a substrate (preferably a glass substrate, although other materials may be used), an underlayer provided above the substrate, an exchange layer structure including a ferromagnetic layer and a nonmagnetic coupling layer, which exchange layer structure is provided above the underlayer, and a magnetic recording layer provided above the exchange layer structure. The ferromagnetic layer is preferably composed of CoCrPtB containing approximately 21 to 23 at % Cr, approximately 10 to 14 at % Pt, and approximately 3 to 5 at % B, with the remainder being Co. The magnetic recording layer is preferably composed of CoCrPtBCu containing approximately 18 to 20 at % Cr, approximately 10 to 12 at % Pt, approximately 6 to 8 at % B, and approximately 4 to 5 at % Cu, with the balance being Co.
Preferably, the medium also includes an NiP layer with an oxide film on its surface, whereby the NiP layer is provided above the substrate. The medium also preferably includes a non-magnetic intermediate layer above the underlayer.
Preferably, an adhesive layer containing Cr as a main constituent is intermediately provided between the substrate and the NiP layer. The underlayer may be divided into a first underlayer and a second underlayer, with the first underlayer being provided on the second underlayer. The first underlayer preferably contains Cr as a main constituent, and the second underlayer preferably contains CrMo as a main constituent.
Preferably, the film thickness of the first underlayer is in the range of approximately 2 to 5 nm, the film thickness of the second underlayer is in the range of approximately 2 to 6 nm, and the total film thickness of the first and second underlayers is in the range of approximately 5 to 10 nm.
Preferably, the substrate has a surface that is textured so as to have a multiplicity of grooves extending in the circumferential direction. The textured surface preferably has an average roughness of not more than 0.4 nm, a number of the grooves of not less than 15 per xcexcm, with the average depth of the grooves being not more than 2 nm.
In accordance with another aspect of the present invention, there is provided a method of producing a magnetic recording medium including the steps of: forming an NiP layer on a substrate in a first vacuum sub-chamber, oxidizing the NiP layer in a second vacuum sub-chamber, forming an underlayer on the oxidized NiP layer in a third vacuum sub-chamber, forming a nonmagnetic intermediate layer having an hcp structure on the underlayer in a fourth vacuum sub-chamber, forming a ferromagnetic layer on the nonmagnetic intermediate layer in a fifth vacuum sub-chamber, forming a nonmagnetic coupling layer on the ferromagnetic layer in a sixth vacuum sub-chamber, and forming a magnetic recording layer on the nonmagnetic coupling layer in a seventh sub-vacuum chamber. Preferably, at least the steps conducted in at least the first, second and third vacuum sub-chambers are all performed without breaking the vacuum. More preferably, all of the steps are performed without breaking the vacuum. Additionally, the ferromagnetic layer is preferably composed of CoCrPtB containing approximately 21 to 23 at % Cr, approximately 10 to 14 at % Pt, and approximately 3 to 5 at % B, with the remainder being Co. Finally, the magnetic recording layer is preferably composed of CoCrPtBCu containing approximately 18 to 20 at % Cr, 10 to 12 at % Pt, 6 to 8 at % B, and 4 to 5 at % Cu, with the remainder being Co.
Preferably, the formation and oxidation of the NiP layer are conducted at a temperature of not less than 140xc2x0 C. Also, the method of producing a magnetic recording medium preferably includes the step of subjecting the surface of the substrate to texturing so as to provide a multiplicity of grooves in the circumferential direction. The surface of the glass substrate preferably has an average roughness of not more than 0.4 nm, a number of the grooves of not less than 15 per xcexcm, and an average depth of the grooves of not more than 2 nm. Preferably, the step of oxidizing the NiP layer is conducted under an oxygen gas partial pressure in the range of approximately 0.1 to 0.6 Pa.
In accordance with a further aspect of the present invention, there is provided a method of producing a magnetic recording medium including the steps of: forming an NiP layer above a substrate while introducing oxygen into a first vacuum sub-chamber, forming an underlayer above the NiP layer in a second vacuum sub-chamber, forming a ferromagnetic layer above the underlayer in a third vacuum sub-chamber, and forming a magnetic recording layer above the ferromagnetic layer in a fourth vacuum sub-chamber. The ferromagnetic layer is preferably composed of CoCrPtB containing approximately 21 to 23 at % Cr, approximately 10 to 14 at % Pt, and approximately 3 to 5 at % B, with the remainder being Co. The magnetic recording layer is preferably composed of CoCrPtBCu containing approximately 18 to 20 at % Cr, approximately 10 to 12 at % Pt, approximately 6 to 8 at % B, and approximately 4 to 5 at % Cu, the remainder being Co.
Preferably, the step of forming the NiP layer is conducted in a gas mixture of argon and oxygen containing not less than approximately 5% of oxygen. Preferably, the formation of the NiP layer is conducted at a temperature of not less than approximately 140xc2x0 C.